Electric discharge tube



21, 1 A. J. w. M. VAN OVERBEEK ELECTRIC DISCHARGE TUBE Original Filed July 29, 1948 3 Sheets-Sheet 1 INVENTOR Adriunus Johannes Wilhelmus Marie V n Overbeek Aug. 21, 1956 A. J. w. M. VAN OVERBEEK ELECTRIC DISCHARGE TUBE Original Filed July 29, 1948 5 Sheets-Sheet 2 as ZskhZs/EE M 0 M Z V00: 0/152 V0!!! 0072 5 .49 Fig.2

INVENTOR Adricnus Johannes Wilhelmus Marie Von Overbeek By WAWQGM' Aug. 21, 1956 A. J. w. M. VAN OVERBEEK ELECTRIC DISCHARGE TUBE Original Filed July 29, 194 s Sheets-Sheet s i ii --I I "E 5 .10 H :5, --1H Zz' "I "1 ll "I I o n E 7 7l NVENTOR Adrionus Johannes Wilhe mus or Va Overbeek Agent United States Patent C) T ELECTRIC DISCHARGE TUBE Adrianus Johannes Wilhelrnus Marie Van Overbeek, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn, as trustee Continuation of abandoned application Serial No. 41,365,

July 29, 1948. This application October 15, 1951, SerialNo. 251,351

Claims priority, application Netherlands September 3, 1947 2 Claims. (Cl. 250-27) This application is a continuation of application Number 41,365, filed July 29, 1948, now abandoned.

This invention relates to a device for detecting frequencymodulated signals comprising an electric discharge tube, to the control grids of which tube two auxiliary signals produced from an incoming signal are applied. The strength of these auxiliary signals depends upon the intensity of the incoming signal. These auxiliary signals also have a phase-displacement relative to each other which is proportional to the deviations in frequency of the incoming signal from a definite center frequency. The electric discharge tube comprises at least four grids including the two control grids, to each of which oneof the auxiliary signals is supplied and each of which is adapted to pass or check completely and anode current of given strength (saturation current).

Such a device is known per se from U. S. A. patent specification 2,343,263, wherein a multigrid-tube is used and the saturated anode-current is obtained byapplying a definite, fixed, positive potential to the first grid, 1

reckoned from the cathode, the second and fourth grid being used as control grids and the third grid being a screen-grid. The fifth and, as the case may be, sixth grid act as second screen grid and as suppressor grid.

The customary attempts are directed to choosing the setting of such a tube or constructing the tube in such manner as to render the mutual conductance of the two control-grids as high as possible at the working point. Thus it is achieved that the device operates satisfactorily even with weak incoming signals.

It has, however, been found that a serious disadvantage may occur in operating. a device comprising a tube of this type. In tuning to the desired signal it appears in effect that this is often very difficult, since the sound strength may increase or may be the same in tuning beyond the desired signal.

As a criterion it then holds that upon correct tuning, at which the signal may have a lower sound strength than incorrect tuning, the distortion is at a minimum and consistently increases with incorrect tuning. Consequently, the ear does not permit of correct tuning; In the present case the provision of simple tuning means, as is usual in receiving amplitude-modulated signals, is not possible without the need of further means, so that sometimes a separate circuit comprising an amplitudemodulation detector or other artifices would have to be resorted to, which involves complications and is consequently expensive. The advantages obtained with this known device, namely the obtainment of detection and limiting eifect by means of a single tube and consequently independency of the sound strength from the intensity of the incoming signal, are often offset by the aforesaid phenomenon, if additional circuits and tubes were not provided for ensuring correct tuning.

This disadvantage is practically entirely avoided if, ac-

cording to' the invention, in an electric discharge tube Patented Aug. 21, 1956 Ice grids, suitable for use in a device for detecting, frequencymodulated signalgforwhich purpose two auxiliarysignals are produced, of which the strength depends upon the intensity of the incoming signal, whilst the two auxiliary signals exhibit a relative phase-displacement proportionalv to the deviations of the frequency of the incoming, signal from a given frequency, which auxiliary signals are eachsupplied to a control-grid of the tube, and. for which purpose a saturated electron-current is produced in the tube, which current can be passed or suppressed completely by each control grid under the influence of the auxiliary signals, the mutual conductance of the two control grids of the tube in the proximity of the working point is low, and more particularly lower than the saturated anode current expressed in milliamps divided by 4 volts. This is obtained to a high degree by winding, the control-grids with a variable pitch, these two grids being preferably arranged in the tube and constructed in such manner that at least two parts of the second controlgrid which exhibit openings of different sizes with respect to one another, are located opposite a part of the first control grid exhibiting openings of a definite size. A decrease in mutual conductance is also obtainable by making use of inverse feedback from the anode circuit of the tube. This means may be used in combination with the particular construction of the tube. I

For obtaining such a low mutual conductance according to the invention it is very advantageous if the length of the cathode over which it is coated with electronemitting material is small e; g. 1 to 10 mms. The positive voltages applied to the remaining screen-grids present in the" tube should likewise be comparatively 10WviZiAO volts at the' utmost.

T (I obtain a marked saturated anode-current an accelerator grid is placed between the cathode and the firstj control grid, which accelerator grid may be connected to a screen grid and is consequently also supplied with a positive voltage of 40 volts at the most. this first grid is preferably less than 5 times the wire diartie'ter.

Between the second control-grid and the anode there may be provided a suppressor grid which may be connected to the cathode. Alternatively, the pitch of this suppressor gridmay be chosen to be so great that the positive anode field engages through this grid in such manner that a positive potential is set up in the plane of this grid so that a separate positive screen-grid is no longer required between the second control-grid and the anode.

With the use of the invention it appears that tuning of" the aforesaid device tothe correct wavelength of the frequency modulated signal no longer entails difficulties. The position of the tuning pointer in which the signal weakens at both sides is determined and the device is then set at the middle of these two limits just as in the usual receiving sets for amplitude-modulated signals, which comprise automatic gain control.

The invention is based on the following realizat'ion' which will beexplained with reference to the accompany ing drawings, given byway of example.

In this drawing Fig. 1 represents a circuit-arrangement;

Figs. 2, 3, 4, 5, 6', 7, 8 and 9 show graphs illustrating the operation of a known device and of a device accordingto the invention.

Fig. 10 shows the arrangement of the two control grids.

Fig. 11 is a cross-section of a discharge tube, and- Figs. 12 and 1-3 are detail views of such a tube which is par-- ticularly suitable for use in a device according. to the invention.

In Fig. 1 the reference numeral 1 designates the receiving device of the frequency-modulated incoming signal, by which device the incoming signal is converted into two The pitch of auxiliary voltages on the conductors 2 and 3 which, for example, with non-modulated incoming signals, exhibit a phase-displacement of 90. With a fully modulated incoming signal this phase-displacement of the auxiliary signals varies, for instance, between 60 and 120. These auxiliary signals are supplied to the two control grids of tube 4 and the low-frequency modulation is separated and further amplified at 5. The control grids are connected, through the conductor 6, to the chassis by way of the coupling member in the receiving device 1. Alternatively, inverse feedback from the anode lead of tube 4 may be used to reduce the mutual conductance. To this end the conductor 6 is connected to a point of a voltage divider 41 placed between the anode and the chassis, as is indicated in Fig. 1.

The tube 4 operates with a saturated anode current so that the control voltages 9 and (Fig. 2) enclosing the cross-hatched area 8, produce current pulses 8' in the anode circuit, which pulses are independent of the amplitude of the signals 9 and 10 if this amplitude has a sufiiciently high value. Consequently the average strength of the anode current depends upon the duration of the pulses 8' passed simultaneously by the two control grids. In this event the tube should be set exactly at the center (point 7) of the control range.

when the above noted condition prevails the saturated anode current is 1.2 milliamp. With no incoming signal, the direct anode current will be /2 x 1.2 milliamps., corresponding to the grid bias of point 7 (Fig. 2). As shown in Fig. 3 with a non-modulated incoming signal the auxiliary signals which are displaced in phase by 90 will consequently cause the anode current to be halved again, resulting in the passage of an average anode current of /2 /z l.2 milliamp.=0.3 milliamp. (line 11). Upon modulation of the incoming signal, when this phase-displacement changes between 60 and 120", the average value of the anode-current passed becomes 0.2 milliamp. and 0.4 milliamp. respectively (lines 12 and 13). In the case of exact adjustment of the grid-control voltages at point 7 in Fig. 2 the signal strength, upon de-tuning of the device, will indeed decrease to zero according to lines 12' and 13, if the strength of the auxiliary signals falls below a definite value, in the present instance 3.5 volts effective. The steep parts 12 and 13' of lines 12 and 13 detect amplitude-modulation, but since these slopes are small the detected signal is but small. Consequently, one determines at both sides of correct tuning where the output signal weakens and tunes to the middle of this range just as in tuning to an amplitude-modulated signal with a set comprising automatic gain-control.

In practice, however, it proves extremely diflicult to obtain and maintain grid-control adjustment at point 7 in Fig. 2, notably in mass-production of such apparatus. In effect, even a very small deviation of the resistance from the theoretically correct value has serious consequences, as appears from Fig. 4. Since normal tubes have a comparatively high control-grid slope, the Ia-Vg characteristic curve 14 in Fig. 4 will be comparatively steep which is apparently favourable with a view to suppressing the saturated anode-current as abruptly as possible. However, this results in that, if due to normal tolerances in the circuit elements, one does not adjust to point 7 but, for instance, to point 15 the average anode-current flowing with a non-modulated incoming signal is not A: X /2 I max., but X 4 I max.=% I max.=0.675 milliamp. Consequently the characteristic lines 11, 12 and 13 in Fig. 3 will, for small signals, exhibit the form designated 11", 12 and 13". Owing to the steep slope of lines 12" and 13 strong amplitude-modulation detection will occur with a weak incoming signal so that upon detuning, the signal may even become louder than with correct tuning and, moreover, greatly distorted. It is obvious that this phenomenon is very troublesome and constitutes a great impediment to correct tuning.

the IaVg characteristic curve in Fig. 4 will exhibit the form of line 16. With the same incorrectness of the control-grid voltage adjustment, the device will, for instance, be set to point 15. In this event the deviation of the anode-current deviation is small with respect to the anode current upon correct adjustment. Consequently, the aforesaid phenomenon is liable to occur only to a small degree so that no troublesome amplitude-modulation detection will occur. In this respect an Ia-Vg characteristic curve according to line 17 in Fig. 5, with which the mutual conductance at both sides of the working point is zero over a given area, is very advantageous. This may, for instance, be achieved by means of twoparts control grids, each part of each control-grid supplying one half of the saturated anode current. Any distortion of the anode-current pulses does not affect the quality of the modulation, since this distortion is of a high-frequency nature so that it is removed from the lowfrequency signal. Hence, a deviation of the adjustment of the control-grid bias from 15" to 15', does not affect the operation of the tube.

To obtain the desired straight characteristic lines exhibiting a small slope, for instance line 16 in Fig. 4, it is desirable that the variable pitch should be distributed in a definite manner over the length of the control-grids. Moreover, the construction of the second control-grid should be closely matched to that of the first one. A grid having a pitch gradually varying throughout its length would yield a curved characteristic as shown in Fig. 6, wherein both the bend at the lower end and that at the top of the characteristic curve is large. order to achieve, despite the small slope, that the desired effect is obtained even with comparatively weak signals the attempts will have to be directed to making the bends as acute as possible as indicated by line 16 in Fig. 4. To this end the parts with the largest and with the smallest openings of each of the control grids must be longer, whereas the parts exhibiting openings of medium size must be shorter.

Furthermore it appears that the construction and the position of the second control-grid with respect to the first one must be such that at least two parts of the length of the second control grid, exhibiting openings of different size one with respect to the other, are located opposite a part of the length of the first control grid which exhibits openings of a given size. The purpose thereof appears from Figures 7, 8 and 9 and is clearly set forth below.

Fig. 7 shows a number of Ia-Vgz characteristic curves of the first control-grid g2 at different voltages on the second control grid g4. The characteristic curve a is obtained at zero potential or a positive potential of the second control grid, and with ever increasing negative potentials of g4, the lines b, c, d and e must be obtained to ensure a satisfactory operation of the device. If two variably wound control grids are placed one behind the other in such manner that the largest openings of the two grids face each other, similarly to the smallest openings, the small openings of the second control-grid cannot afiect any longer when the two control-grids become negative, that part of the saturated anode-current which has already been suppressed by the small openings of the first control grid, so that at the beginning the total anode current passed no longer decreases when the second control grid is rendered negative. The position of begin point P of the characteristic curves, at which the control of the largest openings is strongest, remains substantially unchanged, but due to the aforesaid eflFect the characteristic However, in I curves b, c, d, and e at negative grid voltages flow together to a greater or less degree along the line a, at point P as shown in Fig. 8. If, in contradistinction thereto, the control grids are arranged in such manner that the largest openings of one control grid face the smallest openings of the other control grid, the point P will considerably shift for the diiierent characteristic curves, as is shown in Fig. 9, since the anode current part passed by the larger openings of the first control-grid, will be strongly suppressed, by the part of the second control grid exhibiting the smallest apertures and lying behind the said larger openings, when this second control grid becomes negative. Consequently, the anode current tends to flow only at a lesser negative voltage on the first control grid. Now the starting points of lines a", b, c, d and e" are consequently widely spaced apart.

A very suitable construction and position of the two control grids is shown in Fig. 10, in which the length of the two control grids g2 and g4 is plotted vertically, the parts exhibiting different openings being designated I to IV. From this it appears that the second control grid exhibits a comparatively complicated distribution of its openings. Alternatively, a somewhat simpler distribution may be chosen, provided that a part exhibiting openings of a given size of the first control-grid is faced by at least two parts with diflferent openings of the second control-grid.

A tube according to the invention may be constructed as follows (Fig. 11).

A cathode 18 coated with electron-emitting material over a length of 1 to mms. is surrounded by four or more grids 19, 21, 22, 23, an anode 24 and, a suppressor grid 25. The first grid 19 is oval so that the supporting rods are more spaced from the cathode than the active part of this grid. The remaining grids may also be oval, and the anode may be circular-cylindrical. Between the rods 20 and the cathode 18, rods 26 as shown in Fig. 12 are provided which are preferably connected to the cathode. The accelerator grid 19 is connected to the screen grid 22 and given a low positive potential of less than 40 volts, the rod-shaped electrode 26 reducing the current to the accelerator grid 19. The reduction of current might also be achieved if the cathode 18, according to Fig. 13, is coated with electron-emitting material 30 only opposite the active grid parts and the parts of the supporting rods 20 of the accelerator grid 19 facing the cathode are coated with insulating material 31.

For obtaining the very low mutual conductance the following electrode-construction may be used.

Example I With the use of a cathode sprayed over a comparatively great length (8 to 10 mms.):

First grid (accelerator grid) oval: 3.3 x 1.8 mm. diameter,

wire diameter 80;, pitch 0.2 mm.

First control-grid, variably wound 40% of the length, pitch 0.8 mm. (I in Fig. 10)

20% of the length, pitch 0.61 mm. (II in Fig. 10)

% of the length, pitch 0.43 mm. (III in Fig. 10)

25% of the length, pitch 0.2 (IV in Fig. 10)

First screen-grid, between the two control-grids, normal second control-grid, as shown in Fig. 10, but with a slightly greater pitch in connection with a greater crosssection.

Example 11 Short cathode (sprayed over a length of 4.5 mms.) cathode diameter 0.85:

First grid (accelerator grid) oval: 3.3 x 1.8 mms. diameter,

Wire diameter pitch 0.2 mm.

Second grid (first control-grid) oval, diameter 5.3 x 3.1

mms.,

Wire diameter 60p, pitch variable of 0.4 to 1.2 mm.,

approximately as indicated above.

Third grid (screen grid) oval, diameter 7.3 x 4.6 mms.,

wire diameter 80,44, pitch 0.45 mm.

Fourth grid (second control-grid) oval, diameter 9.6 x 7.2

mms.,

Wire diameter 80 pitch variable of 0.45 to 1.3 mm, as

indicated in Example I.

Fifth grid (suppressor grid, connected to cathode) oval, diameter 11.6 X 11.2 mms., wire diameter pitch 0.5 mm, anode diameter 1.3 mm.

It will be appreciated that the invention is not limited to the above examples.

What I claim is:

1. In a circuit arrangement for detecting a frequency modulated wave having frequency deviations about a given center frequency, comprising an input circuit including means to derive from said wave first and second signals, said second signal having a phase displacement relative to the first signal proportional to the deviation of said Wave from said center frequency, an electric discharge tube exhibiting a mutual conductance lower than its saturated anode current in milliamperes divided by four volts at the working point thereof and comprising means including a cathode, an accelerating anode, a first control grid, a screen grid, a second control grid and an anode arranged in that order to produce an anode current of given saturation value in said tube, means applying one of said signals to the first control grid and the other of said signals to the second control grid, said first control grid having a winding with at least two different pitches each extending over a given length portion thereof, said second control grid having at least two portions radially aligned with each given length portion of said first control grid, said two portions of said second control grid having windings of dilferent pitch, each of said first and second control grids being adapted to produce current cut-off in said tube, an output circuit coupled to said anode, and degenerative feedback means coupled between said output and input circuits.

2. In a circuit arrangement for detecting a frequency modulated wave as claimed in claim 1 in which the means coupled between said output and input circuits is a voltage divider, said arrangement including means applying a voltage of not more than 40 volts to said accelerating and screen grids.

References Cited in the file of this patent UNITED STATES PATENTS 2,050,474 Stone Aug. 11, 1936 2,086,271 Jobst et al. July 6, 1937 2,090,218 Miller Aug. 17, 1937 2,134,407 Jonker Oct. 25, 1938 2,145,368 OBrien Jan. 31, 1939 2,228,980 Steimel et al Jan. 14, 1941 2,235,498 Herold Mar. 18, 1941 2,286,337 Bruck June 16, 1942 2,343,263 Okrent Mar. 7, 1944 2,379,764 Thomas July 3, 1945 2,441,254 Stromeyer May 11, 1948 2,460,062 Charton Ian. 25, 1949 

